Use SplitMix

System/Random.hs

@@ -31,10 +31,7 @@
 --   instance of 'Random' allows one to generate random values of type
 --   'Float'.
 --
--- This implementation uses the Portable Combined Generator of L'Ecuyer
--- ["System.Random\#LEcuyer"] for 32-bit computers, transliterated by
--- Lennart Augustsson.  It has a period of roughly 2.30584e18.
---
+-- This implementation uses the SplitMix algorithm [1].
 -----------------------------------------------------------------------------
 
 #include "MachDeps.h"
@@ -77,10 +74,12 @@ module System.Random
 
 import Prelude
 
+import Control.Arrow (first)
 import Data.Bits
 import Data.Int
 import Data.Word
 import Foreign.C.Types
+import qualified System.Random.SplitMix as SM
 
 #ifdef __NHC__
 import CPUTime		( getCPUTime )
@@ -178,23 +177,10 @@ class RandomGen g where
 class SplittableGen g where
 #endif
    -- |The 'split' operation allows one to obtain two distinct random number
-   -- generators. This is very useful in functional programs (for example, when
-   -- passing a random number generator down to recursive calls), but very
-   -- little work has been done on statistically robust implementations of
-   -- 'split' (["System.Random\#Burton", "System.Random\#Hellekalek"]
-   -- are the only examples we know of).
+   -- generators.
    split    :: g -> (g, g)
 
 {- |
-The 'StdGen' instance of 'RandomGen' has a 'genRange' of at least 30 bits.
-
-The result of repeatedly using 'next' should be at least as statistically
-robust as the /Minimal Standard Random Number Generator/ described by
-["System.Random\#Park", "System.Random\#Carta"].
-Until more is known about implementations of 'split', all we require is
-that 'split' deliver generators that are (a) not identical and
-(b) independently robust in the sense just given.
-
 The 'Show' and 'Read' instances of 'StdGen' provide a primitive way to save the
 state of a random number generator.
 It is required that @'read' ('show' g) == g@.
@@ -208,42 +194,18 @@ instance of 'StdGen' has the following properties:
 * It guarantees to consume only a finite portion of the string. 
 
 * Different argument strings are likely to result in different results.
-
 -}
 
-data StdGen 
- = StdGen !Int32 !Int32
+type StdGen = SM.SMGen
 
-instance RandomGen StdGen where
-  next  = stdNext
-  genRange _ = stdRange
+instance RandomGen SM.SMGen where
+  next  = SM.nextInt
 
 #ifdef ENABLE_SPLITTABLEGEN
-instance SplittableGen StdGen where
+instance SplittableGen SM.SMGen where
 #endif
-  split = stdSplit
-
-instance Show StdGen where
-  showsPrec p (StdGen s1 s2) = 
-     showsPrec p s1 . 
-     showChar ' ' .
-     showsPrec p s2
-
-instance Read StdGen where
-  readsPrec _p = \ r ->
-     case try_read r of
-       r'@[_] -> r'
-       _   -> [stdFromString r] -- because it shouldn't ever fail.
-    where 
-      try_read r = do
-         (s1, r1) <- readDec (dropWhile isSpace r)
-	 (s2, r2) <- readDec (dropWhile isSpace r1)
-	 return (StdGen s1 s2, r2)
-
-{-
- If we cannot unravel the StdGen from a string, create
- one based on the string given.
--}
+  split = SM.splitSMGen
+
 stdFromString         :: String -> (StdGen, String)
 stdFromString s        = (mkStdGen num, rest)
 	where (cs, rest) = splitAt 6 s
@@ -256,21 +218,10 @@ generator, by mapping an 'Int' into a generator. Again, distinct arguments
 should be likely to produce distinct generators.
 -}
 mkStdGen :: Int -> StdGen -- why not Integer ?
-mkStdGen s = mkStdGen32 $ fromIntegral s
+mkStdGen s = SM.mkSMGen $ fromIntegral s
 
-{-
-From ["System.Random\#LEcuyer"]: "The integer variables s1 and s2 ... must be
-initialized to values in the range [1, 2147483562] and [1, 2147483398]
-respectively."
--}
 mkStdGen32 :: Int32 -> StdGen
-mkStdGen32 sMaybeNegative = StdGen (s1+1) (s2+1)
-      where
-	-- We want a non-negative number, but we can't just take the abs
-	-- of sMaybeNegative as -minBound == minBound.
-	s       = sMaybeNegative .&. maxBound
-	(q, s1) = s `divMod` 2147483562
-	s2      = q `mod` 2147483398
+mkStdGen32 s = SM.mkSMGen $ fromIntegral s
 
 createStdGen :: Integer -> StdGen
 createStdGen s = mkStdGen32 $ fromIntegral s
@@ -286,10 +237,9 @@ Minimal complete definition: 'randomR' and 'random'.
 class Random a where
   -- | Takes a range /(lo,hi)/ and a random number generator
   -- /g/, and returns a random value uniformly distributed in the closed
-  -- interval /[lo,hi]/, together with a new generator. It is unspecified
-  -- what happens if /lo>hi/. For continuous types there is no requirement
-  -- that the values /lo/ and /hi/ are ever produced, but they may be,
-  -- depending on the implementation and the interval.
+  -- interval /[lo,hi]/, together with a new generator. For continuous types
+  -- there is no requirement that the values /lo/ and /hi/ are ever produced,
+  -- but they may be, depending on the implementation and the interval.
   randomR :: RandomGen g => (a,a) -> g -> (a,g)
 
   -- | The same as 'randomR', but using a default range determined by the type:
@@ -448,12 +398,6 @@ instance Random CDouble where
   -- random rng = case random rng of 
   -- 	         (x,rng') -> (realToFrac (x::Double), rng')
 
-mkStdRNG :: Integer -> IO StdGen
-mkStdRNG o = do
-    ct          <- getCPUTime
-    (sec, psec) <- getTime
-    return (createStdGen (sec * 12345 + psec + ct + o))
-
 randomBounded :: (RandomGen g, Random a, Bounded a) => g -> (a, g)
 randomBounded = randomR (minBound, maxBound)
 
@@ -510,39 +454,6 @@ randomIvalDouble (l,h) fromDouble rng
 int32Count :: Integer
 int32Count = toInteger (maxBound::Int32) - toInteger (minBound::Int32) + 1  -- GHC ticket #3982
 
-stdRange :: (Int,Int)
-stdRange = (1, 2147483562)
-
-stdNext :: StdGen -> (Int, StdGen)
--- Returns values in the range stdRange
-stdNext (StdGen s1 s2) = (fromIntegral z', StdGen s1'' s2'')
-	where	z'   = if z < 1 then z + 2147483562 else z
-		z    = s1'' - s2''
-
-		k    = s1 `quot` 53668
-		s1'  = 40014 * (s1 - k * 53668) - k * 12211
-		s1'' = if s1' < 0 then s1' + 2147483563 else s1'
-
-		k'   = s2 `quot` 52774
-		s2'  = 40692 * (s2 - k' * 52774) - k' * 3791
-		s2'' = if s2' < 0 then s2' + 2147483399 else s2'
-
-stdSplit            :: StdGen -> (StdGen, StdGen)
-stdSplit std@(StdGen s1 s2)
-                     = (left, right)
-                       where
-                        -- no statistical foundation for this!
-                        left    = StdGen new_s1 t2
-                        right   = StdGen t1 new_s2
-
-                        new_s1 | s1 == 2147483562 = 1
-                               | otherwise        = s1 + 1
-
-                        new_s2 | s2 == 1          = 2147483398
-                               | otherwise        = s2 - 1
-
-                        StdGen t1 t2 = snd (next std)
-
 -- The global random number generator
 
 {- $globalrng #globalrng#
@@ -564,7 +475,7 @@ getStdGen  = readIORef theStdGen
 
 theStdGen :: IORef StdGen
 theStdGen  = unsafePerformIO $ do
-   rng <- mkStdRNG 0
+   rng <- SM.initSMGen
    newIORef rng
 
 -- |Applies 'split' to the current global random generator,
@@ -588,22 +499,10 @@ getStdRandom f = atomicModifyIORef' theStdGen (swap . f)
 
 {- $references
 
-1. FW #Burton# Burton and RL Page, /Distributed random number generation/,
-Journal of Functional Programming, 2(2):203-212, April 1992.
-
-2. SK #Park# Park, and KW Miller, /Random number generators -
-good ones are hard to find/, Comm ACM 31(10), Oct 1988, pp1192-1201.
-
-3. DG #Carta# Carta, /Two fast implementations of the minimal standard
-random number generator/, Comm ACM, 33(1), Jan 1990, pp87-88.
-
-4. P #Hellekalek# Hellekalek, /Don\'t trust parallel Monte Carlo/,
-Department of Mathematics, University of Salzburg,
-<http://random.mat.sbg.ac.at/~peter/pads98.ps>, 1998.
-
-5. Pierre #LEcuyer# L'Ecuyer, /Efficient and portable combined random
-number generators/, Comm ACM, 31(6), Jun 1988, pp742-749.
-
-The Web site <http://random.mat.sbg.ac.at/> is a great source of information.
+1. Guy L. Steele, Jr., Doug Lea, and Christine H. Flood. 2014. Fast splittable
+pseudorandom number generators. In Proceedings of the 2014 ACM International
+Conference on Object Oriented Programming Systems Languages & Applications
+(OOPSLA '14). ACM, New York, NY, USA, 453-472. DOI:
+https://doi.org/10.1145/2660193.2660195
 
 -}

cabal.project

@@ -0,0 +1,2 @@
+packages: .
+constraints: splitmix -random

random.cabal

@@ -36,7 +36,7 @@ Library
         System.Random
     extensions:	CPP
     GHC-Options: -O2
-    build-depends: base >= 3 && < 5, time
+    build-depends: base >= 3 && < 5, time, splitmix
 
 source-repository head
     type:     git

shell.nix

@@ -0,0 +1,8 @@
+with import <nixpkgs> {};
+
+mkShell {
+  buildInputs = [
+    cabal-install
+    haskell.compiler.ghc822
+  ];
+}